Netclient Download
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I suggest going to your wow folder and doing the repair thing they have set up in there.
It will either fix the problem or (more likely) make you restart from an older patch.
Another good thing to do is Delete you WTF folder, for some reason this fixes a lot of problems
If you want to continue trying an XP repair install, Michael Stevens Tech has all the details http://michaelstevenstech.com/XPrepairinstall.htm
An alternative method that you could use that is simple and automatic but costs a little is to use Reimage. It completes the repair in about 35 minutes and all your programs and data remain intact. The details of this process are here http://www.askthecomputerdoc.com/blog/reimage-repair/